1. Field of the Invention
In general, this invention relates to infusion catheters. More specifically, this invention relates to an infusion catheter for delivering fluid into an organism where the catheter has a non-reactive lining and tip, surrounded partially by a flexible silicone-type material.
2. Description of the Related Art
When chronic administration of a pharmaceutically active agent is required, internal delivery by an external infusion pump or an implantable infusion pump ("IIP"), in combination with a catheter, may be the desired delivery means. For example, IIP-catheter delivery may be preferred when, for example, the site specific delivery of the drug is critical, or the drug must be administered in tightly controlled, yet minute dosages.
In applications where the quantity of delivered drug is relatively minute and must be carefully tailored, it may be critical that the delivered drug be non-reactive with the material of the catheter. Non-reactive means that the delivered drug flows from the IIP to the delivery site without adhering to, diffusing through, or otherwise chemically reacting with, the catheter itself. Standard delivery catheters normally comprise a single tubular member and are composed of a flexible elastomeric material, typically silicone, that is biocompatible with the animal body into which the desired agent is delivered. A more recent design includes an internal lining that is more compatible with the agent desired to be delivered while maintaining the biocompatibility of the external catheter sheath.
FIG. 1 depicts a portion of a typical prior art implantable catheter 10 with a drug compatible internal lining. Catheter 10 comprises a tubular jacket or sheath 12 that is coupled at one end to the IIP (not shown) and terminates at its other end in a rounded tip 14. One or more elution holes 16 are disposed in sheath 12 proximate the rounded tip 14. Sheath 12 is ordinarily tubular and manufactured from a flexible biocompatible elastomeric material such as silicone. It is desirable for sheath 12 to be both flexible and biocompatible. A flexible material makes catheter 10 easier to conform to the various curved passageways in the body during placement and use. The biocompatibility of sheath 12 will enable catheter 10 to remain in the body for prolonged periods of time without prompting an immune system response. The interior of sheath 12 is lined with a tubular lining 18 which is coextensive with sheath 12 from the IIP (not shown) to a sheath/lining seal point 20. Catheter 10 may not be manufactured solely of this material because the material may often be too rigid to make a usable catheter for actual use. Sheath/lining seal point 20 is ordinarily located a few millimeters from the elution holes 16. The lining 18 is ordinarily fabricated from a material that will be non-reactive with the delivered agent such as polyethylene, polyurethane or polytetraflouroethylene (PTFE) or TEFLON.RTM. as it is commonly known in the trade.
If the delivered drug is sensitive to the material of sheath 12, the delivered drug may either be adsorbed by sheath 12, diffuse across sheath 12 or react chemically with sheath 12 or with substances diffusing through sheath 12 from outside catheter 10. For example, if the delivered drug is adsorbed by sheath 12, the amount of delivered drug may be significantly less than the required dosage. Similarly, if undesirable agents diffuse through sheath 12 and react with the delivered drug, the amount and efficacy of the delivered drug may be compromised. Because the seal point 20 is directly exposed to the delivered drug, there is the potential for undesirable seepage of the drug between sheath 12 and lining 18.
Many drugs or agents exhibit some detrimental sensitivity to silicone or drugs or agents that may diffuse through a silicone sheath. Insulin presents one example. In certain circumstances, carbon dioxide may diffuse through a silicone sheath. If insulin is flowing through catheter 10, carbon dioxide from outside catheter 10 may diffuse through sheath 12 and react with the buffer in the insulin solution, causing a pH change in the insulin solution. As a result, the insulin buffer breaks down, causing degradation and polymerization of the insulin to occur. In some applications involving the chronic dispensing of insulin, suitable buffers to counteract the pH changes brought on by CO.sub.2 diffusion are simply not feasible.
Another example of drugs sensitive to silicone is presented by neurotrophic factors such as nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), ciliary neurotrophic factor (CNTF), or glial-derived neurotrophic factor (GDNF), currently being studied as potential therapies for amyotrophic lateral sclerosis, Parkinson's Disease, or other neurological disorders. The particular neurotrophic factor may either be adsorbed by the interior surface of the silicone sheath 12, or alternatively react with the silicone sheath 12 and degrade into secondary components. The dosage levels for such neurotrophic factors may be so small that an appreciable loss or degradation of the delivered agent will adversely effect the agent's ability to satisfactorily treat the patient.
In the prior art catheter 10 shown in FIG. 1, extension of the lining 18 past the elution holes 16 has proved to be impractical, since it has been difficult, if not impossible, to adequately seal the interfaces between sheath 12 and lining 18 that are in fluid communication with the elution holes 16. The present invention is directed to solving one or more of the above-noted problems.